Patent US 4,454,510

An aircraft surveillance, navigation, landing, guidance and collision avoidance system is provided which takes the form of a two-way data communication link between each of a plurality of controlled aircraft and a ground station, and appropriate measuring equipment at the ground station which provides three-dimensional position data on the ground for air traffic control purposes; and which also provides three-dimensional position data in the aircraft for navigation, landing guidance and collision avoidance purposes, as well as a two-way air/ground data link for miscellaneous data and control purposes. The system of the invention comprises the combination of a ground transmitter/receiver station and airborne transponders mounted in the aircraft. The ground transmitter broadcasts discrete aircraft addressed interrogation pulses which are coded to contain position and/or other data. The transponders in all aircraft receive and decode the interrogation pulses, and the discretely addressed aircraft transmits appropriately timed reply pulses to the ground receiver which contain altitude and other data. Other aircraft utilize the received position information for collision avoidance purposes. The ground receiver measures the incident angles of the reply signal pulses and their precise time delays with respect to the interrogation pulses, so as to provide aircraft position and other data to the ground station, and to provide additional data for transmission to the aircraft on the following interrogation pulses.